Electrolyte for rechargeable lithium battery and rechargeable lithium battery comprising same

ABSTRACT

Disclosed is an electrolyte for a rechargeable lithium battery including: a first additive having an oxidation potential of 4.1 to 4.6V; a second additive having an oxidation potential of 4.4 to 5.0V; a non-aqueous organic solvent; and a lithium salt.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and is based on Korean patentapplication No. 10-2003-0076913 filed in the Korean Industrial PropertyOffice on Oct. 31, 2003 and Korean patent application No.10-2004-0065773 filed in the Korean Industrial Property Office on Aug.20, 2004, the entire contents of both of which are incorporated hereintoby reference.

NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

The subject matter of this application was developed pursuant to a jointresearch agreement between Samsung SDI Co., Ltd. and Cheil Industries,Inc.

FIELD OF THE INVENTION

The present invention relates to an electrolyte for a rechargeablelithium battery and a rechargeable lithium battery comprising the same.More particularly, it is directed to an electrolyte for a rechargeablelithium battery capable of preventing the battery from beingovercharged, providing good cycle-life characteristics to the battery,and permitting safe storage of a battery when stored at hightemperature. It is still further directed to a rechargeable lithiumbattery having such properties.

DESCRIPTION OF THE RELATED ART

The use of portable electronic instruments is increasing as electronicequipment gets smaller and lighter due to developments in high-techelectronic industries. Studies on lithium secondary batteries areactively being pursued in accordance with the increased need forbatteries having high energy density for use as power sources in theseportable electronic instruments. For responding to the demands for highvoltage and high energy density, rechargeable batteries havingnon-aqueous electrolytes, positive active materials of lithium-includedmetal oxides having a voltage of 4V, and negative active materials ofcarbonaceous materials capable of intercalating and deintercalating thelithium have been suggested.

The non-aqueous electrolyte for the rechargeable battery is required forsafety. Particularly, for a rechargeable lithium ion battery, thelithium metal may be deposited on the surface of the negative electrodeor the lithium may be occluded in the negative electrode at more than adesirable amount when the rechargeable lithium ion battery isovercharged, for example, where there are problems with the chargecontrol circuit. In such a situation, too many lithium ions aretransferred from the positive electrode to the negative electrode. Thiscauses the internal resistance of the battery to increase as the batterycontinues to be charged and generates a large amount of heat. In theworst case, it may cause thermal runaway.

In order to prevent such an extraordinary exothermal phenomenon, batterysafety can be enhanced by mounting a thermo-sensitive current disconnectswitch such as a PTC (positive temperature coefficient thermistor)outside of the battery in order to ensure disconnection of the current.Further, in order to solve the overcharge problems, U.S. Pat. No.4,943,497 discloses means for stopping the charge current as soon as achange of internal pressure is sensed in the battery. However, it isdifficult to recoup the cost of a mechanical current disconnectmechanism, and it is also difficult to insert such a mechanical deviceinside of a battery, especially considering the tendency toward smallerand thinner batteries.

Accordingly, Japanese Patent laid-open publication Nos. H01-206571,H06-338347, and H07-302614 disclose a method of self-consuming theelectric energy with a redox shuttle by adding an additive causing areversible redox reaction in an electrolyte. However, this method cannotensure safety since the electric charge transferring reaction speed andthe transferring speed of lithium ions are limited when the overchargecurrent increases. On this point, Japanese Patent laid-open publicationNos. H09-50822, H 10-50342, and H10-321258, U.S. Pat. No. 2,939,469, andlaid-open publication No. 2000-58117 disclose that an aromatic compoundhaving a methoxyl group and a halogen atom, biphenyl, thiophen, or anaromatic ether compound is added to an electrolyte, which polymerizes,thereby stopping the temperature from further increasing to order tosecure safety. Further, Japanese Patent laid-open publication Nos.H09-106835, H09-171840 and H10-321258 disclose that biphenyl,3-R-thiopen, 3-chlorothiopen, and furane are polymerized duringovercharge, thereby securing safety.

However, an additive such as biphenyl has shortcomings in that it maygradually decompose when the charge and discharge voltage is not uniformor when an increase in voltage occurs unexpectedly and quickly. Theamount of the additive also tends to gradually decrease when the batteryis discharged at a high temperature of more than 40° C. Such an additiveis generally unable to ensure battery safety after 300 charge anddischarge cycles and it also deteriorates the cycle-lifecharacteristics. Furthermore, such an additive has poor high-temperaturestorage characteristics. Such problems are more severe as the amount ofadditive increases (Japanese Patent laid-open publication No.H11-162512).

To address the deterioration in the cycle-life characteristicsassociated with the use of biphenyl, it is proposed to use2,2-diphenlypropane in Japanese Patent laid-open publication No.H11-162512. However, 2,2-dipehenylpropane only slightly improves thecycle-life characteristics, and does not provide as high a level ofsafety as a biphenyl. Most significantly, with the use of the2,2-diphenylpropane, the cycle-life characteristics deterioratesignificantly.

SUMMARY OF THE INVENTION

In one embodiment of the present invention an electrolyte is providedfor a rechargeable lithium battery exhibiting good safety withoutthermal runaway during overcharging and having good cycle-lifecharacteristics, good high-temperature storage characteristics, and goodstability.

In another embodiment of the invention a rechargeable lithium battery isprovided that includes such an electrolyte.

In one embodiment, an electrolyte is provided for a rechargeable lithiumbattery including a first additive having an oxidation potential of 4.1to 4.6V, a second additive having an oxidation potential of 4.4 to 5.0V,a non-aqueous organic solvent, and a lithium salt. The first additive ispreferably a compound represented by Formula 2 below, and the secondadditive is preferable a compound represented by Formula 5 below:

-   -   wherein R5 is a C₁-C₆alkyl group or a C₂-C₆ alkylene group.

-   -   wherein X is Cl, Br or F, and R1 is hydrogen, a C₁-C₆ alkyl        group or an alkylene group.

The oxidation potential for the electrolyte is preferably between 4.4and 4.8V.

The present invention also provides a rechargeable lithium batteryincluding the electrolyte, a positive electrode, and a negativeelectrode. The positive electrode and the negative electrode includeactive materials capable of reversibly intercalating and deintercalatinglithium ions. At least one of the positive electrode and the negativeelectrode includes a water-soluble binder and a water-soluble agent forincreasing viscosity.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic view showing a rechargeable lithium batteryaccording to the present invention;

FIG. 2 is a graph showing LSV decomposition start voltages of cellsaccording to Examples 1 to 3 and Comparative Example 1;

FIG. 3 is a graph showing LSV decomposition start voltages of cellsaccording to Examples 4 to 7 and Comparative Example 1;

FIG. 4 is a graph showing LSV decomposition start voltages of cellsaccording to Examples 8 to 10 and Comparative Example 1; and

FIG. 5 is a graph showing LSV data when the additive represented byFormula 5 was mixed to the additive represented by Formula 2.

DETAILED DESCRIPTION

The present invention relates to an electrolyte for a rechargeablelithium battery that is capable of providing the battery with improvedsafety upon an overcharge condition, and good high temperature storageand cycle-life characteristics.

In order to ensure safety in the event of overcharge, the electrolyte ofthe present invention includes at least two different kinds of organiccompounds having HOMO (Highest Occupied Molecular Orbital) values higherthan those of conventional carbonates. The at least two differentorganic compounds also have different oxidation potential values fromeach other.

In one embodiment, the electrolyte of the present invention includes afirst additive having a low oxidation potential of 4.1 to 4.6V (referredto herein as a “lower potential-overcharge additive”), and a secondadditive having a high oxidation potential of 4.4 to 5.0V (referred toherein as a “higher potential-overcharge additive”). The second additivehas an oxidation potential higher than that of the first additive. Forexample, the first additive is selected to have an oxidation potentialof lower than 4.4V when the oxidation potential of the second additiveis 4.4V, and the second additive is selected to have an oxidationpotential of higher than 4.6V when the oxidation potential of the firstadditive is 4.6V.

More preferably, the higher potential-overcharge additive has anoxidation potential of 4.6 to 5.0V, and most preferably 4.7 to 4.9V. Thelower potential-overcharge additive more preferably has an oxidationpotential of 4.1 V to 4.6V, and most preferably 4.2V to 4.4V. Further,the potential difference between the higher potential-overchargeadditive and the lower potential-overcharge additive is in a range ofbetween 0.1 and 0.7V, and preferably between 0.3 and 0.7V.

Further, the electrolyte which includes the higher potential-overchargeadditive and the lower potential-overcharge additive preferably has anoxidation potential of 4.4 to 4.9V, and more preferable 4.5 to 4.7V whenthe electrolyte includes a carbonate-based solvent as an organicsolvent. Such an oxidation potential may be varied depending on the kindof organic solvents used in the electrolyte and thus, as the standardsolvent, the present invention employs a carbonate-based solvent as anorganic solvent used in the electrolyte. However, it is not limited tothe organic solvent used in the electrolyte. Therefore, regardless ofthe kind of organic solvent used in electrolytes of batteries, when anelectrolyte includes the inventive higher and lower potential-overchargeadditives, it is expected to have the oxidation potential if thecarbonate-based organic solvent is used as the organic solvent.

Oxidation potential is the potential at which an oxide reaction startsand a voltage at which decomposition begins. The voltage at whichdecomposition starts indicates a potential at which the current valuevariation of 0.00001 A/cm2 begins at a room temperature of 20 to 25° C.under an air atmosphere. The amount of the additive is 1% or more byweight based on the total weight of the electrolyte. If the amount isless than 1% by weight, the peak value changing the current value isdifficult to monitor using the naked eye.

The HOMO (Highest Occupied Molecular Orbital) is the outermost (highestenergy) orbital containing electrons. Accordingly, if any molecule is tolose an electron, it would most likely lose it from the outermostorbital. A material with a high HOMO value oxidizes easily, but amaterial with a low HOMO value is difficult to oxidize. All additivesused in the present invention have a HOMO value of between −9 and −8 eV,which is higher than that of between −12 and −11 eV of thecarbonate-based organic solvent. These values are determined accordingto PM3 (Parametric Method Number 3) of the semi-empirical molecularorbital calculating method.

The lower potential-overcharge additive may include at least onecompound represented by the following Formulae 1 and 2:

-   -   wherein R1 is hydrogen, a C₁-C₆ alkyl group, or a C₂-C₆ alkylene        group; R4 is a C₁-C₆ alkyl group, —OOCR6 (wherein R6 is        hydrogen, a C₁-C₆ alkyl group, a C₂-C₆ alkylene group), or —OR7        (wherein R7 is hydrogen, a C₁-C₆ alkyl group or a C₂-C₆ alkylene        group); and R5 is a C₁-C₆ alkyl group or a C₂-C₆ alkylene group.

Exemplary compounds represented by Formula 1 may include, for example,compounds represented by Formulae 3 and 4:

-   -   where R1 is defined as for Formula 1; R6 is hydrogen, a C₁-C₆        alkyl group or a C₂-C₆ alkylene group; and R7 is hydrogen, a        C₁-C₆ alkyl group or a C₂-C₆ alkylene group.

In addition, the higher potential-overcharge additive may include atleast one of compounds represented by one of Formulae 5 to 9:

wherein X is Cl, Br, or F; R1 is a C₁-C₆ alkyl group or an alkylenegroup; R2 is a C₁-C₆ alkyl group, an alkoxy group, or an alkylene group;R3 is a C₁-C₆ alkyl group or a C₂-C₆ alkylene group; R4 is selected fromthe group consisting of C₁-C₆ alkyl groups, —OOCR6 where R6 is selectedfrom the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆alkylene groups, and —OR7 where R7 is selected from the group consistingof hydrogen, C₁-C₆ alkyl groups and C₂-C₆ alkylene groups; and A is SO₂,CH₂ C═O, or CF₂.

Exemplary compounds represented by the Formulae 6 to 8 includes, forexample, compounds represented by Formulae 10 to 13:

wherein, X is Cl, Br, or F; and R3 is a C₁-C₆ alkyl group or a C₂-C₆alkylene group.

Most preferably, the lower potential-overcharge additive is a compoundrepresented by Formula 2, and the higher potential-overcharge additiveis a compound represented by Formula 5.

If only a lower potential-overcharge additive is used, it is generallyadded in an amount of 1% by weight or more to obtain the safetyimprovement effects during overcharge. However, such an additive maypartially react with the active material to coat the surface of theactive material, thereby dramatically deteriorating the batteryperformance when the battery is placed at a high temperature (80-90°C.). Further, as the charge and discharge cycles are repeated, thevolumes of the negative electrode and the positive electrode arerepeatedly constricted and expanded to cause local imbalances of thenegative electrode and the positive electrode. Thereby, the partialdifference of the potential may cause deterioration of the cycle-lifecharacteristics of the battery since the additive having a low oxidepotential intended to improve safety during overcharging decomposesearlier than others. Further, when only a higher potential-overchargeadditive is used, it may cause problems in that the safety during theovercharge is not fully ensured since the reactive potential of theadditive is too high during overcharge.

The electrolyte according to the present invention including both thefirst additive and the second additive at an appropriate ratio. Thiscombination ensures safe operation during overcharge, while stillproviding good cycle-life and good high-temperature storage andstability characteristics. According to the present invention, two ormore overcharge additives having different oxidation potentials are usedas the overcharge additive. Specifically, the additive having the lowoxidation potential is added in very small amounts to maintain goodstability of storage at high temperatures. The additive having arelatively higher oxidation potential ensures safety by reacting earlierduring overcharge. Therefore, both recovery on storage and safety duringovercharge can be satisfied, which are not achieved when only one of theadditives is used.

According to the present invention, the electrolyte preferably includes0.01 to 20% by weight of the higher potential-overcharge additive andthe lower potential-overcharge additive combined, more preferably 0.1 to10% by weight, and most preferably 5 to 10% by weight. When the combinedamounts of the higher potential-overcharge additive and the lowerpotential-overcharge additive are less than 0.01% by weight, it isdifficult to achieve satisfactory effects during overcharge. When thecombined amount is more than 20% by weight, the cycle-lifecharacteristics deteriorate remarkably.

The weight ratio of the higher potential-overcharge additive to thelower potential-overcharge additive is preferably in the range of 20:0.1to 3:2, and more preferably 10:0.1 to 4:1. On calculating the weightratio, the amount of the lower potential-overcharge additive is 0.01 to5% by weight in the electrolyte, and that of the higherpotential-overcharge additive is 1 to 20% by weight.

As mentioned above, the compound represented by Formula 2 is preferablyused for the lower potential-overcharge additive, and the compoundrepresented by Formula 5 is preferably used for the higherpotential-overcharge additive. The weight ratio of the higherpotential-overcharge additive to the lower potential-overcharge additiveis preferably between 10:0.1 to 0.2, more preferably between 7.5:0.1 to0.2, and most preferably between 5:0.1 to 0.2. When the mixing ratio ofthe higher potential-overcharge additive and the lowerpotential-overcharge additive are present in the preferred range,overcharge safety is improved as are the cycle-life characteristics andthe recovery characteristics of a battery at high temperature storage.

The electrolyte of the present invention further includes a non-aqueousorganic solvent and a lithium salt. The non-aqueous organic solvent actsa medium capable of transmitting ions taking part in the electrochemicalreaction of the battery. The non-aqueous organic solvent may include acarbonate, ester, ether, or ketone. The carbonate may include, but isnot limited to, dimethyl carbonate, diethyl carbonate, dipropylcarbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethylcarbonate, ethylene carbonate, propylene carbonate, butylene carbonate,and so on. The ester may include, but is not limited to,y-butyrolactone, n-methyl acetate, n-ethyl acetate, n-propyl acetate andso on. The ester may be dibutyl ether and the ketone may bepolymethylvinyl ketone. The carbonate solvent of the non-aqueous organicsolvent is preferably prepared by mixing a cyclic carbonate and a chaincarbonate. Preferably, the cyclic carbonate is mixed with the chaincarbonate at a volume ratio of between 1:1 and 1:9. The electrolyteproperties can be improved when the mixing ratio of the cyclic carbonateand the chain carbonate are within this range.

The non-aqueous organic solvent may further include an aromatichydrocarbon organic solvent. In this case, a carbonate organic solventis preferred. The aromatic hydrocarbon organic solvent may includecompounds represented by Formula 14:

-   -   wherein, R9 is a halogen or a C₁-C₁₀ alkyl group and n is an        integer between 0 and 6. The aromatic hydrocarbon organic        solvent may include, but is not limited to benzene,        fluorobenzene, chlorobenzene, nitro benzene, toluene,        fluorotoluene, trifluorotoluene, xylene, and so on. The        electrolyte including the aromatic hydrocarbon organic solvent        is preferably composed of a carbonate solvent/aromatic        hydrocarbon solvent in a volume ratio of between 1:1 and 30:1.        The properties of the electrolyte can be improved when the        volume ratio is in this range.

The lithium salt acts as a source for supplying lithium ions in thebattery, and helps the battery operation. The non-aqueous organicsolvent acts as a medium for the ions which take part in theelectrochemical reaction. The lithium salts may include, but are notlimited to, at least one of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄,CF₃SO₃Li, LiN(SO₂CF₃)₂, LiC₄F₉SO₃, LiAlO₄, LiAlOCl₄, LiN(SO₂C₂F₅)₂),LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) (wherein x and y are naturalnumbers), LiCl, and Lil.

The concentration of the lithium salt is preferably between 0.1 and2.0M. When the concentration of the lithium salt is less than 0.1M, theconductivity of the electrolyte is decreased, deteriorating theelectrolyte properties. When the concentration is more than 2.0M, theviscosity of the electrolyte may increase, decreasing the transmittanceof lithium ions.

The rechargeable lithium battery according to the present inventionincludes the electrolyte of the present invention, a negative electrodeand a positive electrode. The positive electrode includes a positiveactive material capable of reversibly intercalating and deintercalatinglithium ions. The positive active material may include lithiatedintercalation oxides.

The negative electrode may include a negative active material capable ofintercalating and deintercalating lithium ions. The negative activematerial may include a carbonaceous negative active material of acrystalline or amorphous carbon, or a carbon complex (for example,thermally decomposed carbon, cokes, graphite), a fired organic polymercompound, carbon fiber, a zinc oxide compound, lithium metal or alithium alloy. The preferred negative active material is a crystallinecarbon, and more preferably a graphite having an Lc (crystallite size)of 150Å or more and preferably between 150 and 3000Å, a d(002) ofbetween 3.35 and 3.38Å of X-ray diffraction plane distance at the (002)plane, a true density of 2.2 g/cm³ or more and preferably between 2.2and 2.3 g/cm³, a BET (specific surface area) of between 0.5 and 50 m²/g,and an average particle diameter (D50) of between 1 and 30 μm.Furthermore, the negative active material preferably has an intensityratio Ra I(I(1360 cm⁻¹)/I(1580 cm⁻¹)) of a Raman Spectroscopy peakintensity I(1360) at a (1360) plane to an Raman Spectroscopy peakintensity I(1590) at a (1590) plane of 0.05 or more, and morepreferably, between 0.05 and 0.5. The negative active material of thepresent invention preferably has an X-ray diffraction peak intensityratio I(110)/(002) of an X-ray diffraction peak intensity (110) at a(110) plane to an X-ray diffraction peak intensity I(002) at a (002)plane of 0.2 or less, and more preferably between 0.006 and 0.2.

The battery according to the present invention may further include abinder. The binder both helps to bind the active material particles toone another and to bind the positive active material to a currentcollector. The binder may include any material used a the conventionalrechargeable lithium battery. Examples include those soluble in organicsolvents or in water. Binders soluble in organic solvents include, butare not limited to, polytetrafluoro ethylene polyvinylidene fluoride,polyethylene, polypropylene, polyvinyl chloride, polyvinyl pyrrolidone,and polyvinyl alcohol.

Binders soluble in water include, but are not limited to,styrene-butadiene rubber, polysodium acrylate, propylene and C2-8 olefincopolymer and (meth)acrylic acid and (meth)acrylic alkyl estercopolymers.

Binders soluble in water may further include a water-soluble agent forincreasing viscosity in order to increase the binding property. Thewater-soluble agent for increasing viscosity may include acellulose-based compound. The cellulose compound may include, but is notlimited to, carboxyl methyl cellulose, hydroxyl propyl cellulose,hydroxyl propyl methyl cellulose, hydroxyl ethyl cellulose, hydroxylpropyl ethyl cellulose, or methyl cellulose. In addition, it may includean alkaline metal salt thereof. The alkaline metal salt may include analkali metal such as Na, K, or Li. When the cellulose-based compoundwith an alkaline metal salt is used, the discharge property of thebattery at a high rate is improved more than by using only acellulose-based compound without the alkaline metal.

The rechargeable lithium battery of the present invention may furtherinclude a separator for preventing a short between the positiveelectrode and the negative electrode. The separator may include anyconventional material such as a polymer membrane of polyolefin,polypropylene, polyethylene, or other polymer materials, or amulti-layer polymer membrane, a micro-porous film, or a fabric ornon-fabric membrane.

The rechargeable lithium battery including the electrolyte, the positiveelectrode, the negative electrode, and the separator has a structuresuch as a unit cell of positive electrode-separator-negative electrode,a bicell of positive electrode-separator-negativeelectrode-separator-positive electrode, or a lamination cell such thatthe unit cell is repeated.

A representative lithium battery of the present invention is shown inFIG. 1. FIG. 1 shows a cylindrical lithium ion battery 1 including anegative electrode 2; a positive electrode 3; a separator 4 interposedbetween the negative electrode 2 and the positive electrode 3. Anelectrolyte is injected around the negative electrode 2, the positiveelectrode 3, and the separator 4 which are placed within a batterycontainer 5 with a sealing member 6 for sealing the battery container 5.However, the rechargeable lithium battery is not limited to thestructure shown in FIG. 1, and can be varied to any shape such as aprismatic, or a pouch shaped battery.

The following examples further illustrate the present invention, but theinvention is not to be limited by these examples.

EXAMPLES 1 TO 10 AND COMPARATIVE EXAMPLE 1

An electrolyte solution was prepared by dissolving 1M LiPF6 in a mixedsolvent of ethylene carbonate and ethylmethyl carbonate (in a 1:1 volumeratio). To this was added 2% by weight of an additive representedFormulae 1 to 10 as shown in Table 1 to provide electrolyte solutionsfor Examples 1 to 10, respectively. Comparative Example 1 is anelectrolyte solution including 1M LiPF6 dissolved in the mixed solventof ethylene carbonate and methylethyl carbonate (in a 1:1 volume ratio)without the additive.

The resulting electrolyte solutions were measured for oxidationpotential (Linear Sweep Voltametry:LSV) by an electrochemical analyzer(Potensiostat Model273A, EG&G) at room temperature (25° C.). The workingelectrode was made of a platinum electrode having a diameter of 1 mm,and the reference electrode and the counter electrode were made oflithium. The voltage range was between 3 and 7V and the scan rate was 1mV/s. The values obtained were rounded to the three decimal places. Thetests were carried out at room temperature of 25° C. in a sealed dryroom. The results show that compounds represented by Formulae 2 to 5 and10 to 13 of Examples 1 to 10 are decomposed earlier than that of thecarbonate organic solvent of Comparative Example 1. The decompositionvoltages are defined as where the potential starting the change at0.00001 A/cm2, are shown in the following Table 1. Further, HOMOs ofExamples 1 to 10 and Comparative Example 1 are shown in the followingTable 1. The HOMOs were determined by PM3 (Parametric Method Number 3)of the semi-empirical molecular orbital calculation method.

TABLE 1 Electrolyte (1 M LiPF₆) adding with ethylenecarbonate:ethylmethyl LSV carbonate = 1:1, in HOMO decomposition amountof 2 wt. % value (eV) start voltage (V) Comparative — Ethylene 5.64Example 1 carbonate (−11.78), ethylmethyl carbonate (−11.52) Example 14-chlorotoluene −9.25 4.73 (Formula 5) Example 2 4-bromotoluene −9.524.73 (Formula 5) Example 3 4-fluorotoluene −9.49 4.70 (Formula 5)Example 4 4,4′-(hexafluoro −9.58 4.42 (Formula 10) isopropylidene)diphenol Example 5 4-phenyl-1,3-dioxane −9.75 4.80 (Formula 11) Example6 4,4-cyclohexylidene −8.88 4.41 (Formula 12) bisphenol Example 7bisphenol S(4,4′- −9.70 4.80 (Formula 13) sulphonyl diphenol) Example 8bisphenol A diacetate −9.17 4.26 (Formula 3) Example 9 bisphenol A −8.894.25 (Formula 4) Example 10 1,1-diphenyl ethylene −9.04 4.26 (Formula 2)

As shown in Table 1, the compounds represented by Formulae 2 to 5 and 10to 13 started to decompose earlier than that of the carbonate-basedorganic solvent, they had lower LSV decomposition start voltages, andthe oxide reactions occurred at the LSV decomposition start voltages.

Further, the compounds of Examples 1 to 3 and Comparative Example 1 weremeasured for LSV decomposition start voltage, and the results are shownin FIG. 2. The results of LSV decomposition start voltage for Examples 4to 7 and Comparative Example 1 are shown in FIG. 3, and those ofExamples 8 to 10 and Comparative Example 1 are shown in. FIG. 4. Asshown in FIGS. 2 to 4, the electric current is remarkably increased atLSV decomposition start voltage. The figures also indicate thatcompounds of Examples 1 to 10, that is, compounds represented byFormulae 2 to 5 and 10 to 13 have LSV decomposition start voltages lowerthan that of Comparative Example 1.

After raising the battery voltages to 4.2V, they were overcharged to theovercharge state, then the compounds represented by the above formulaestarted to decompose and to generate gas. At the same time, thepolymerization began, generating the polymerizing heat, and theseparator began to shut down due to the heat. In this condition, whenthe overcharge was further continued, the gas generating amount wasincreased, then the vent was opened after 30-40 minutes from the startof the overcharge and the overcharge current was blocked. The batterytemperature then slowly decreased. Therefore, the additive can ensurebattery safety during overcharge compared to a battery without theadditive.

EXAMPLE 11

Artificial graphite negative active material was suspended in an aqueoussolution of carboxyl methyl cellulose, and a styrene-butadiene rubberbinder was added thereto to prepare a negative active material slurry.The obtained slurry was coated on a copper foil with a thickness of 10μm, dried and compressed to produce a negative electrode. The artificialgraphite negative active material had an Lc of 3540Å, a d(002) of3.357Å, a true density of 2.22 g/cm3, a BET (specific surface area)value of 2.5 m²/g, and an average particle diameter (D50) of 18 μm. Inaddition, the negative active material had an intensity ratio Ra I(1360cm−1)/I(1580 cm−1) of a Raman spectroscopy peak intensity I(1360) at a(1360) plane to an Raman spectroscopy peak intensity I(1590) at aI(1590) plane of 0.27, and an X-ray peak intensity ratio I(110)/(002),of 0.055.

A LiCoO₂ positive active material, a polyvinylidene fluoride binder, anda carbon conductive agent were dispersed at a weight ratio of 92:4:4 ina solvent of N-methyl-2-pyrrolidone to prepare a positive activematerial slurry. The positive active material was coated on an aluminumfoil with a thickness of 15 μm, dried and compressed to produce apositive electrode.

Together with a 16 μm thick polyethylene separator, the producedelectrodes were wound and pressed, then placed into prismatic canshaving the dimensions of 46 mm×34 mm×50 mm. An electrolyte was injectedinto the cans, thereby completing the fabrication of the rechargeablelithium battery cells. For an electrolyte, LiPF₆ was added to a mixedsolvent of ethylene carbonate and ethylmethyl carbonate (in a 1:1 volumeratio) to be 1.0M to prepare a mixture, and 2.0% by weight of4-chlorotoluene and 0.2% by weight of bisphenol A diacetate based on thetotal weight of the mixture were added to the mixture.

EXAMPLE 12

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-chlorotoluene and 0.2% byweight of bisphenol A based on the total weight of the mixture wereadded to the mixture.

EXAMPLE 13

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-chlorotoluene and 0.2% byweight of 1,1-diphenylethylene based on the total weight of the mixturewere added.

EXAMPLE 14

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-bromotoluene and 0.2% byweight of bisphenol A diacetate based on the total weight of the mixturewere added.

EXAMPLE 15

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-bromotoluene and 0.2% byweight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 16

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-bromotoluene and 0.2% byweight of 1,1-diphenylethylene based on the total weight of the mixturewere added.

EXAMPLE 17

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-fluorotoluene and 0.2% byweight of bisphenol A diacetate based on the total weight of the mixturewere added.

EXAMPLE 15

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-fluorotoluene and 0.2% byweight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 19

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-fluorotoluene and 0.2% byweight of 1,1-diphenylethylene based on the total weight of the mixturewere added.

EXAMPLE 20

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of4,4′-(hexafluoroisopropylidene)diphenol and 0.2% by weight of bisphenolA diacetate based on the total weight of the mixture were added.

EXAMPLE 21

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of4,4′-(hexafluoroisopropylidene)diphenol and 0.2% by weight of bisphenolA based on the total weight of the mixture were added.

EXAMPLE 22

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of4,4′-(hexafluoroisopropylidene)diphenol and 0.2% by weight of1,1-diphenylethylene based on the total weight of the mixture wereadded.

EXAMPLE 23

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-phenyl-1,3-dioxane and 0.2%by weight of bisphenol A diacetate based on the total weight of themixture were added.

EXAMPLE 24

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-phenyl-1,3-dioxane and 0.2%by weight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 25

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4-phenyl-1,3-dioxane and 0.2%by weight of 1,1-diphenylethylene based on the total weight of themixture were added.

EXAMPLE 26

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4,4-cyclohexylidene bisphenoland 0.2% by weight of bisphenol A diacetate based on the total weight ofthe mixture were added.

EXAMPLE 27

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4,4-cyclohexylidene bisphenoland 0.2% by weight of bisphenol A based on the total weight of themixture were added.

EXAMPLE 28

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of 4,4-cyclohexylidene bisphenoland 0.2% by weight of 1,1-diphenylethylene based on the total weight ofthe mixture were added.

EXAMPLE 29

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of bisphenol S (4,4′-sulfonyldiphenol) and 0.2% by weight of bisphenol A diacetate based on the totalweight of the mixture were added.

EXAMPLE 30

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of bisphenol S (4,4′-sulfonyldiphenol) and 0.2% by weight of bisphenol A based on the total weight ofthe mixture were added.

EXAMPLE 31

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 2.0% by weight of bisphenol S (4,4′-sulfonyldiphenol) and 0.2% by weight of 1,1-diphenylethylene based on the totalweight of the mixture were added.

EXAMPLE 32

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.50% by weight of 4-chlorotoluene and 1.0% byweight of bisphenol A diacetate based on the total weight of the mixturewere added.

EXAMPLE 33

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-chlorotoluene and 0.01% byweight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 34

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-chlorotoluene and 0.05% byweight of 1,1-diphenylethylene based on the total weight of the mixturewere added.

EXAMPLE 35

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of 4-bromotoluene and 1.0% byweight of bisphenol A diacetate based on the total weight of the mixturewere added.

EXAMPLE 36

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-bromotoluene and 0.01% byweight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 37

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-bromotoluene and 0.05% byweight of 1,1-diphenylethylene based on the total weight of the mixturewere added.

EXAMPLE 38

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of 4-fluorotoluene and 1.0% byweight of bisphenol A diacetate based on the total weight of the mixturewere added.

EXAMPLE 39

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-fluorotoluene and 0.01% byweight of bisphenol A based on the total weight of the mixture wereadded.

EXAMPLE 40

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-fluorotoluene and 0.05% byweight of 1,1-diphenyl ethylene based on the total weight of the mixturewere added.

EXAMPLE 41

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of 4,4′-(hexafluoroisopropylidene) diphenol and 1.0% by weight of bisphenol A diacetatebased on the total weight of the mixture were added.

EXAMPLE 42

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4,4′-(hexafluoroisopropylidene) diphenol and 0.01% by weight of bisphenol A based on thetotal weight of the mixture were added.

EXAMPLE 43

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4,4′-(hexafluoroisopropylidene) diphenol and 0.05% by weight of 1,1-diphenylethylenebased on the total weight of the mixture were added.

EXAMPLE 44

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of 4-phenyl-1,3-dioxane and 1.0%by weight of bisphenol A diacetate based on the total weight of themixture were added.

EXAMPLE 45

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4-phenyl-1,3-dioxane and0.01% by weight of bisphenol A based on the total weight of the mixturewere added.

EXAMPLE 46

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10% by weight of 4-phenyl-1,3-dioxane and 0.05%by weight of 1,1-diphenylethylene based on the total weight of themixture were added.

EXAMPLE 47

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of 4,4-cyclohexylidene bisphenoland 1.0% by weight of bisphenol A diacetate based on the total weight ofthe mixture were added.

EXAMPLE 48

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4,4-cyclohexylidene bisphenoland 0.01% by weight of bisphenol A based on the total weight of themixture were added.

EXAMPLE 49

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 10.0% by weight of 4,4-cyclohexylidene bisphenoland 0.05% by weight of 1,1-diphenylethylene based on the total weight ofthe mixture were added.

EXAMPLE 50

A rechargeable lithium cell was fabricated by the same procedure as inExample 11, except that 1.5% by weight of bisphenol S(4,4′-sulphonyldiphenol) and 1.0% by weight of bisphenol A diacetate based on the totalweight of the mixture were added.

EXAMPLE 51

A rechargeable lithium cell was fabricated by the same procedure as inExample 1 1, except that 10.0% by weight of bisphenol S(4,4′-sulphonyldiphenol) and 0.01% by weight of bisphenol A based on the total weightof the mixture were added.

EXAMPLE 52

A rechargeable lithium cell was fabricated by the same procedure as. inExample 11, except that 10.0% by weight of bisphenol S(4,4′-sulphonyldiphenol) and 0.05% by weight of 1,1-diphenyl ethylene based on thetotal weight of the mixture were added.

COMPARATIVE EXAMPLE 2

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that an additive was not added to theelectrolyte.

COMPARATIVE EXAMPLE 3

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4-chlorotoluene based onthe total weight of the mixture was added.

COMPARATIVE EXAMPLE 4

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4-bromotoluene based onthe total weight of the mixture was added.

COMPARATIVE EXAMPLE 5

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4-fluorotoluene based onthe total weight of the mixture was added.

COMPARATIVE EXAMPLE 6

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4,4′-(hexafluoroisopropylidene) diphenol based on the total weight of the mixture wasadded.

COMPARATIVE EXAMPLE 7

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4-phenyl-1,3-dioxane wasadded based on the total weight of the mixture.

COMPARATIVE EXAMPLE 8

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 4,4-cyclohexylidenebisphenol was added based on the total weight of the mixture.

COMPARATIVE EXAMPLE 9

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of bisphenolS(4,4′-sulphonyl diphenol) was added based on the total weight of themixture.

COMPARATIVE EXAMPLE 10

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of bisphenol A diacetatewas added based on the total weight of the mixture.

COMPARATIVE EXAMPLE 11

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of bisphenol A was addedbased on the total weight of the mixture.

COMPARATIVE EXAMPLE 12

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 2.0% by weight of 1,1-diphenyl ethylenewas added based on the total weight of the mixture.

COMPARATIVE EXAMPLE 13

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 0.2% by weight of bisphenol A diacetatewas added based on the total weight of the mixture.

COMPARATIVE EXAMPLE 14

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 0.2% by weight of bisphenol A was addedbased on the total weight of the mixture.

COMPARATIVE EXAMPLE 15

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that 0.2% by weight of 1,1-diphenyl ethylenewas added based on the total weight of the mixture.

The battery cells (battery capacity 1 C=790 mAh) fabricated by theprocedures of Examples 11 to 52 and Comparative Examples 2 to 15 werecharged at constant current and constant voltage at the electric currentof 158 mA and the charge voltage of 4.2V, and then allowed to stand for1 hour followed by discharging at the electric current of 395 mA to2.75V and being allowed to stand for 1 hour. The procedure was repeated3 times, then the battery cells were charged at the electric current of395 mA and the charge voltage of 4.2V for 3 hours.

For the overcharge test, 20 battery cells of each of Examples 11 to 52and Comparative Examples 2 to 15 were overcharged from the charge stateat room temperature (25° C.) under constant current and constant voltageof 1 C (790 mAh)/12V for 2.5 hours. The battery cell states weremonitored and the results are shown in Tables 2 to 4.

The high temperature storage characteristics were measured by allowingthe charged battery cells to stand at 85° C. for 4 days, and thedischarge capacity at 0.5 C was measured. The retention recovery ratiowas expressed by comparing the discharge capacity at 0.5 C after the 4days at high temperature as a percentage of the discharge capacity at0.5 C before the 4 days at high temperature storage.

For the cycle-life characteristics test, the cells according to Examples11 to 24 and Comparative Example 2 were constant current and constantvoltage charged at 1 C/4.2V under a cut-off of 0.1 C and a cut-offdischarge of 1 C/3.0V. The capacity retention ratio was expressed bycomparing the discharge capacity after 300 cycles as a percentage of thedischarge capacity for 1 cycle.

In addition, cycle-life tests were carried out at the high temperature(45° C.) and at room temperature (25° C.).

In Tables 2 to 4, the numbers before L refer to the number of test cellsand the measuring criterions of the safety during overcharge are definedas follows:

-   -   L0: good, L1: leaking, L2: flashing, L2: sparking, L3: smoking,        L4: burning, L5: exploding.

For example, 15L0, 5L4 means that among a total of 20 cells tested, 15cells were indicated as good, and 5 cells were burning.

TABLE 2 First additive Capacity (lower Second additive LSV Retentionretention Capacity potential- (higher potential- decomposition recoveryrate at 45° C. for retention at room overcharge overcharge startingvoltage at high 300 cycles temperature for additive) additive) (V)Overcharge temperature (%) 300 cycles (%) Example bisphenol A4-chlorotoluene 4.50 20L0 80 75 85 11 diacetate Example bisphenol A4-chlorotoluene 4.43 20L0 77 74 86 12 Example 1,1-diphenylethylene4-chlorotoluene 4.47 20L0 79 76 85 13 Example bisphenol A 4-bromotoluene4.56 20L0 76 74 87 14 diacetate Example bisphenol A 4-bromotoluene 4.5520L0 78 75 86 15 Example 1,1- 4-bromotoluene 4.40 20L0 77 76 88 16diphenylethylene Example bisphenol A 4-fluorotoluene 4.60 20L0 76 77 8517 diacetate Example bisphenol A 4-fluorotoluene 4.40 20L0 75 74 87 18Example 1,1-diphenylethylene 4-fluorotoluene 4.46 20L0 79 75 85 19Example bisphenol A 4,4′-(hexafluoro 4.32 20L0 75 76 86 20 diacetateisopropylidene) diphenol Example bisphenol A 4,4′-(hexafluoro 4.30 20L073 77 89 21 isopropylidene) diphenol Example 1,1-diphenylethylene4,4′-(hexafluoro 4.35 20L0 74 75 87 22 isopropylidene) diphenol Examplebisphenol A 4-phenyl-1,3- 4.70 20L0 75 76 86 23 diacetate dioxaneExample bisphenol A 4-phenyl-1,3- 4.50 20L0 73 78 87 24 dioxane Example1,1-diphenylethylene 4-phenyl-1,3- 4.60 20L0 76 79 85 25 dioxane Examplebisphenol A 4,4- 4.35 20L0 75 75 88 26 diacetate cyclohexylidenebisphenol Example bisphenol A 4,4- 4.32 20L0 74 76 87 27 cyclohexylidenebisphenol Example 1,1-diphenylethylene 4,4- 4.36 20L0 76 78 86 28cyclohexylidene bisphenol Example bisphenol A bisphenol S(4,4′- 4.4820L0 75 77 85 29 diacetate sulfonyldiphenol) Example bisphenol Abisphenol S(4,4′- 4.40 20L0 73 75 86 30 sulfonyldiphenol) Example1,1-diphenylethylene bisphenol S(4,4′- 4.47 20L0 76 76 86 31sulfonyldiphenol)

TABLE 3 Retention recovery Capacity Capacity retention First additive(lower Second additive rate at the retention at 45° at roompotential-overcharge (higher potential- high for 300 cycles temperaturefor additive) overcharge additive) Overcharge temperature (%) 300 cycles(%) Example bisphenol A diacetate 4-chlorotoluene 20L0 75 74 80 32Example bisphenol A 4-chlorotoluene 20L0 80 73 88 33 Example1,1-diphenylethylene 4-chlorotoluene 20L0 80 75 88 34 Example bisphenolA diacetate 4-bromotoluene 20L0 73 76 80 35 Example bisphenol A4-bromotoluene 20L0 79 77 88 36 Example 1,1-diphenylethylene4-bromotoluene 20L0 78 78 89 37 Example bisphenol A diacetate4-fluorotoluene 20L0 71 75 79 38 Example bisphenol A 4-fluorotoluene20L0 77 76 88 39 Example 1,1-diphenylethylene 4-fluorotoluene 20L0 80 7787 40 Example bisphenol A diacetate 4,4′-(hexafluoro 20L0 70 75 82 41isopropylidene)diphenol Example bisphenol A 4,4′-(hexafluoro 20L0 75 7690 42 isopropylidene)diphenol Example 1,1-diphenylethylene4,4′-(hexafluoro 20L0 77 75 88 43 isopropylidene)diphenol Examplebisphenol A diacetate 4-phenyl-1,3-dioxane 20L0 71 76 88 44 Examplebisphenol A 4-phenyl-1,3-dioxane 20L0 77 74 89 45 Example1,1-diphenylethylene 4-phenyl-1,3-dioxane 20L0 78 73 87 46 Examplebisphenol A diacetate 4,4-cyclohexylidene 20L0 72 75 83 47 bisphenolExample bisphenol A 4,4-cyclohexylidene 20L0 78 76 88 48 bisphenolExample 1,1-diphenylethylene 4,4-cyclohexylidene 20L0 77 77 87 49bisphenol Example bisphenol A diacetate bisphenol S(4,4′- 20L0 71 75 8750 sulfonyldiphenol) Example bisphenol A bisphenol S(4,4′- 20L0 77 76 8751 sulfonyldiphenol) Example 1,1-diphenylethylene bisphenol S(4,4′- 20L078 74 89 52 sulfonyldiphenol)

TABLE 4 Second Capacity additive Retention retention at First additive(higher LSV recovery at Capacity the room (lower potential- potential-decomposition high retention at temperature overcharge overchargestarting voltage temperature 45° C. for 300 for 300 cycles additive)additive) (V) Overcharge (%) cycles (%) (%) Comparative — — 5.64 20L5 9580 91 Example 2 Comparative 4-chlorotoluene — 4.73 15L0, 5L4 90 75 86Example 3 Comparative 4-bromotoluene — 4.73 14L0, 6L4 85 76 85 Example 4Comparative 4-fluorotoluene — 4.70 15L0, 5L4 87 77 88 Example 5Comparative 4,4′-(hexafluoro — 4.42 18L0, 2L4 80 75 85 Example 6isopropylidene) diphenol Comparative 4-phenyl-1,3- — 4.80 10L0, 10L4 8575 89 Example 7 dioxane Comparative 4,4- — 4.41 17L0, 3L4 83 76 84Example 8 cyclohexylidene bisphenol Comparative Bisphenol — 4.48 17L0,3L4 84 77 85 Example 9 S(4,4′-sulfonyl diphenol) Comparative bisphenol A— 4.26 20L0 49 15 38 Example 10 diacetate Comparative bisphenol A — 4.2520L0 45 20 40 Example 11 Comparative 1,1-diphenyl — 4.26 20L0 48 10 25Example 12 ethylene Comparative bisphenol A — 4.26 20L5 94 75 89 Example13 diacetate Comparative bisphenol A — 4.25 20L5 93 76 88 Example 14Comparative 1,1-diphenyl — 4.26 20L5 92 73 88 Example 15 ethylene

As shown in Table 4, when the battery of Comparative Example 2 which didnot include any additive was overcharged, all 20 cells exploded.Conventionally, in order to ensure battery safety, safety mechanismssuch as a charge voltage protection circuit and a current blockingmechanism are installed. However, in the case of Comparative Example 2no safety protection devices were provided, permitting the explosions tooccur.

Further, according to Comparative Example 10, 11 and 12, all of thecells added with bisphenol A diacetate, bisphenol A, or1,1-diphenylethylene for the overcharge additive having the lowoxidation-potential could ensure the safety during overcharge to the L0level, but they had insufficient retention recovery rates at the hightemperature and insufficient capacity retention at 300 cycles, at 50% orless. When the cells were analyzed after standing at the hightemperature, it was determined that the surfaces of the positiveelectrodes were coated with polymer which prevented the charge anddischarge reaction of lithium ions and deteriorated the retentionrecovery rates.

According to Comparative Examples 3 to 9 with additives havingrelatively higher potentials than those of Comparative Examples 10, 11and 12, the results on overcharge characteristics were improved comparedto that of Comparative Example 2, but having L4 or more was not perfectprevention compared to those of Comparative Examples 10, 11 and 12 allhaving L0. However, they improved the retention recovery rate at hightemperature and the cycle characteristics at 80% or more compared tothose of Comparative Examples 10, 11 and 12.

Since the cells according to Comparative Examples 13, 14 and 15 had theadditive having the lower oxidation potential added at a low amount of0.2%, the retention recovery rate at the high temperature and the cyclecharacteristics were improved, but the safety during overcharge was notensured. Thereby, it is understood that the single additive cannotprovide the desired safety during overcharge, good storagecharacteristics at high temperature, and good cycle characteristics.

Unlike the cells of Comparative Examples, the cells of Examples 11 to 52including both the first and the second additives had improved retentionrecovery rate at the high temperature, up to 70% or more, as shown inTables 2 and 3, and the capacity retention at 300 cycles was 79% ormore. Particularly, the cells of Examples 13, 16, and 29 including thecompounds represented by Formula 5 as the higher potential-overchargeadditive and the compound represented by Formula 2 as the lowerpotential-overcharge additive improved the retention recovery rate atthe high temperature. In addition, the additive having the lower LinearSweep Voltametry and slightly reacting during overcharge increased thepolarization during overcharge, so that the additive having the higherLinear Sweep Voltametry started the reaction at the early stage toimprove the safety during overcharge enough to show that all cells wereL0. As a result, batteries having good retention recovery rates at hightemperature and good safety during the overcharge can be achieved.

This is supported by the fact that the LSV decomposition start voltageis between those of the higher additive and the lower additive whenthese additive are mixed. For example, the LSV decomposition startvoltage is 4.47V, which is between 4.26 of the lower additive (Formula2) and 4.73 of the higher additive (Formula 5) when these additives aremixed. FIG. 5 shows LSV data when the additive represented by Formula 5was mixed to the additive represented by Formula 2.

Further, the cells of Comparative Examples 10 to 12 including only thelower potential-overcharge additive at more than a predetermined amounthave good safety during overcharge, but the cycle-life characteristicsare remarkably deteriorated due to the side reaction of the lowerpotential-overcharge additive on the cycle-life at high temperature. Thecells according to Comparative Examples 3 to 9 including only the higherpotential-overcharge additive improved the cycle-life characteristics,but the overcharge property remarkably deteriorated. However, the cellsaccording to Examples 11 to 52 including both the lowerpotential-overcharge additive and the higher potential-overchargeadditive were satisfactory in terms of both the overcharge property andthe cycle-life characteristics at high temperature.

EXAMPLE 53

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a polyvinyl pyrrolidone binder was used, a4-chlorotoluene higher potential-overcharge additive was added in anamount of 5% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 54

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 12, except that a polyvinyl pyrrolidone binder was used, a4-chlorotoluene higher potential-overcharge additive was added in anamount of 5% by weight, and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.2% by weight of the total weight ofthe mixture.

EXAMPLE 55

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 13, except that polyvinyl pyrrolidone was used for thebinder of the negative electrode, 4-chlorotoluene for the higherpotential-overcharge additive was added in an amount of 5% by weight anda 1,1-diphenyl ethylene lower potential-overcharge additive was added inan amount of 0.2% by weight of the total weight of the mixture.

EXAMPLE 56

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 14, except that a polyvinyl pyrrolidone binder, a4-bromotoluene higher potential-overcharge additive was added in anamount of 5% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 57

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 15, except that a polyvinyl pyrrolidone binder was used, a4-bromotolene higher potential-overcharge additive was added in anamount of 5% by weight, and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.2% by weight of the total weight ofthe mixture.

EXAMPLE 58

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 16, except that a polyvinyl pyrrolidone binder was used, a4-bromotoluene higher potential-overcharge additive was added in anamount of 5% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 59

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 17, except that a polyvinyl pyrrolidone binder was used, a4-fluorotoluene higher potential-overcharge additive was added in amountof 5% by weight and a bisphenol A diacetate lower potential-overchargeadditive was added in an amount of 0.2% by weight of the total weight ofthe mixture.

EXAMPLE 60

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 18, except that a polyvinyl pyrrolidone binder was used, a4-fluorotoluene higher potential-overcharge additive was added in anamount of 5% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.2% by weight of the total weight ofthe mixture.

EXAMPLE 61

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 19, except that a polyvinyl pyrrolidone binder was used, a4-fluorotoluene higher potential-overcharge additive was added in anamount of 5% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 62

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 20, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylene)dipheneol higher potential-overchargeadditive was added in an amount of 5% by weight and a bisphenol Adiacetate lower potential-overcharge additive was added in an amount of0.2% by weight of the total weight of the mixture.

EXAMPLE 63

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 21, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylene)dipheneol higher potential-overchargeadditive was added in an amount of 5% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 64

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 22, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylene)dipheneol higher potential-overchargeadditive was added in an amount of 5% by weight and a 1,1-diphenylethylene lower potential-overcharge additive was added in an amount of0.2% by weight of the total weight of the mixture.

EXAMPLE 65

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 23, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 5% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 66

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 24, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 5% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.2% by weight of the total weight ofthe mixture.

EXAMPLE 67

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 25, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 5% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 68

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 26, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential-overcharge additive wasadded in an amount of 5% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 69

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 27, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential-overcharge additive wasadded in an amount of 5% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 70

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 28, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential-overcharge additive wasadded in an amount of 5% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 71

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 29, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 5% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 72

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 30, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 5% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 73

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 31, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 5% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.2% by weightof the total weight of the mixture.

EXAMPLE 74

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 32, except that a polyvinyl pyrrolidone binder was used, a4-chlorotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A diacetate lowerpotential-overcharge additive added in an amount of 0.5% by weight ofthe total weight of the mixture.

EXAMPLE 75

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 33, except that a polyvinyl pyrrolidone binder was used, a4-chlorotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.5% by weight of the total weight ofthe mixture.

EXAMPLE 76

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 34, except that a polyvinyl pyrrolidone binder was used, a4-chlorotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 77

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 35, except that a polyvinyl pyrrolidone binder was used, a4-bromoetoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 78

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 36, except that a polyvinyl pyrrolidone binder was used, a4-bromotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.5% by weight of the total weight ofthe mixture.

EXAMPLE 79

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 37, except that a polyvinyl pyrrolidone binder was used, a4-bromotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 80

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 38, except that a polyvinyl pyrrolidone binder was used, a4-fluorotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A diacetate lowerpotential-overcharge-additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 81

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 39, except that a polyvinyl pyrrolidone binder was used, a4-fluorotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.5% by weight of the total weight ofthe mixture.

EXAMPLE 82

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 40, except that a polyvinyl pyrrolidone binder was used, a4-bromotoluene higher potential-overcharge additive was added in anamount of 10% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 83

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 41, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylidene) diphenol higher potential-overchargeadditive was added in an amount of 10% by weight and a bisphenol Adiacetate lower potential-overcharge additive was added in an amount of0.5% by weight of the total weight of the mixture.

EXAMPLE 84

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 42, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylidene) diphenol higher potential-overchargeadditive was added in an amount of 10% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 85

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 43, except that a polyvinyl pyrrolidone binder was used, a4,4′-(hexafluoroisopropylidene) diphenol higher potential-overchargeadditive was added in an amount of 10% by weight and a 1,1-diphenylethylene lower potential-overcharge additive was added in an amount of0.5% by weight of the total weight of the mixture.

EXAMPLE 86

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 44, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 10% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 87

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 45, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 10% by weight and a bisphenol A lower potential-overchargeadditive was added in an amount of 0.5% by weight of the total weight ofthe mixture.

EXAMPLE 88

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 46, except that a polyvinyl pyrrolidone binder was used, a4-phenyl-1,3-dioxane higher potential-overcharge additive was added inan amount of 10% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 89

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 47, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential overcharge additive wasadded in an amount of 10% by weight and a bisphenol A diacetate lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 90

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 48, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential-overcharge additive wasadded in an amount of 10% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 91

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 49, except that a polyvinyl pyrrolidone binder was used, a4,4-cyclohexylidene bisphenol higher potential-overcharge additive wasadded in an amount of 10% by weight and a 1,1-diphenyl ethylene lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 92

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 50, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 10% by weight and a bisphenol A diacetatelower potential-overcharge additive was added in an amount of 0.5% byweight of the total weight of the mixture.

EXAMPLE 93

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 51, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 10% by weight and a bisphenol A lowerpotential-overcharge additive was added in an amount of 0.5% by weightof the total weight of the mixture.

EXAMPLE 94

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 52, except that a polyvinyl pyrrolidone binder was used, abisphenol S(4,4′-sulfonyl diphenol) higher potential-overcharge additivewas added in an amount of 10% by weight and a 1,1-diphenyl ethylenelower potential-overcharge additive was added in an amount of 0.5% byweight of the total weight of the mixture.

The cells fabricated by procedures of Examples 53 to 94 (batterycapacity 1 C=790 mAh) were charged at an electric current of 158 mA anda charge voltage of 4.2V under constant current and constant voltage andallowed to stand for one hour followed by discharging at the electriccurrent of 395 mA and 2.75V over one hour. These steps were repeatedthree times, and the cells were then charged at the electric current of395 mA and the charge voltage of 4.2V for three hours.

For the overcharge test, 20 cells each of Examples 53 to 94 were chargedat room temperature (25° C.), and then overcharged under the conditionof constant current and constant voltage of 1 C (790 mAh) and 12V for2.5 hours. The cells were monitored and the results are shown in Tables5 and 6.

The high temperature storage characteristics were measured by allowingthe charged cells to stand at 85° C. for 4 days, and the dischargecapacity was measured. The retention recovery rate was expressed bycomparing the discharge capacity at 0.5 C after 4 days at hightemperature as a percentage of the discharge capacity at 0.5 C beforethe 4 days at high temperature.

To test of the cycle-life characteristics, the cells were cut-offcharged at 0.1 and cut-off discharged at 1 C/4.2V under constant currentand constant voltage. The capacity retention was expressed by comparingthe discharge capacity after 300 cycles as a percentage of the dischargecapacity at the first cycle.

In Tables 5 and 6, the number before “L” means the number of test cells,and the condition for measuring the safety during overcharge wasidentical to that of Table 2.

TABLE 5 First additive Second additive LSV Retention Capacity (lowerpotential- (higher potential- decomposition recovery rate at retentionovercharge overcharge initiation high temperature at 300 additive)additive) voltage(V) Overcharge (%) cycles (%) Example bisphenol A4-chlorotoluene 4.50 20L0 83 87 53 diacetate Example bisphenol A4-chlorotoluene 4.43 20L0 75 83 54 Example 1,1- 4-chlorotoluene 4.4720L0 78 83 55 diphenylethylene Example bisphenol A 4-bromotoluene 4.5620L0 78 86 56 diacetate Example bisphenol A 4-bromotoluene 4.55 20L0 7984 57 Example 1,1- 4-bromotoluene 4.40 20L0 76 85 58 diphenylethyleneExample bisphenol A 4-fluorotoluene 4.60 20L0 77 83 59 diacetate Examplebisphenol A 4-fluorotoluene 4.40 20L0 76 86 60 Example 1,1-4-fluorotoluene 4.46 20L0 77 86 61 diphenylethylene Example bisphenol A4,4′-(hexafluoro 4.32 20L0 76 83 62 diacetate isopropylidene)dip Examplebisphenol A 4,4′-(hexafluoro 4.30 20L0 73 82 63 isopropylidene)dipExample 1,1- 4,4′-(hexafluoro 4.35 20L0 75 84 64 diphenylethyleneisopropylidene)dip Example bisphenol A 4-phenyl-1,3- 4.70 20L0 75 87 65diacetate dioxane Example bisphenol A 4-phenyl-1,3- 4.50 20L0 74 87 66dioxane Example 1,1- 4-phenyl-1,3- 4.60 20L0 78 86 67 diphenylethylenedioxane Example bisphenol A 4,4- 4.35 20L0 76 82 68 diacetatecyclohexylidene Example bisphenol A 4,4- 4.32 20L0 74 81 69cyclohexylidene Example 1,1- 4,4- 4.36 20L0 78 82 70 diphenylethylenecyclohexylidene Example bisphenol A bisphenol S(4,4′- 4.48 20L0 75 85 71diacetate sulfonyldiphenol) Example bisphenol A bisphenol S(4,4′- 4.4020L0 78 85 72 sulfonyldiphenol) Example 1,1- bisphenol S(4,4′- 4.47 20L077 86 73 diphenylethylene sulfonyldiphenol)

TABLE 6 Retention recovery rate First additive Second additive at high(lower potential- (higher potential- temperature Capacity retentionovercharge additive) overcharge additive) Overcharge (%)) at 300 cycles(%) Example bisphenol A diacetate 4-chlorotoluene 20L0 76 83 74 Examplebisphenol A 4-chlorotoluene 20L0 79 87 75 Example 1,1-diphenylethylene4-chlorotoluene 20L0 80 88 76 Example bisphenol A diacetate4-bromotoluene 20L0 75 81 77 Example bisphenol A 4-bromotoluene 20L0 7987 78 Example 1,1-diphenylethylene 4-bromotoluene 20L0 76 87 79 Examplebisphenol A diacetate 4-fluorotoluene 20L0 71 79 80 Example bisphenol A4-fluorotoluene 20L0 77 89 81 Example 1,1-diphenylethylene4-fluorotoluene 20L0 82 87 82 Example bisphenol A diacetate4,4′-(hexafluoro 20L0 70 85 83 isopropylidene)diphen Example bisphenol A4,4′-(hexafluoro 20L0 75 90 84 isopropylidene)diphenol Example1,1-diphenylethylene 4,4′-(hexafluoro 20L0 78 88 85isopropylidene)diphen Example bisphenol A diacetate 4-phenyl-1,3-dioxane20L0 71 87 86 Example bisphenol A 4-phenyl-1,3-dioxane 20L0 77 85 87Example 1,1-diphenylethylene 4-phenyl-1,3-dioxane 20L0 78 85 88 Examplebisphenol A diacetate 4,4-cyclohexylidene 20L0 72 83 89 bisphenolExample bisphenol A 4,4-cyclohexylidene 20L0 75 88 90 bisphenol Example1,1-diphenylethylene 4,4-cyclohexylidene 20L0 77 88 91 bisphenol Examplebisphenol A diacetate bisphenol S(4,4′- 20L0 71 86 92 sulfonyldiphenol)Example bisphenol A bisphenol S(4,4′- 20L0 75 87 93 sulfonyldiphenol)Example 1,1-diphenylethylene bisphenol S(4,4′- 20L0 78 88 94sulfonyldiphenol)

COMPARATIVE EXAMPLE 16

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 4-chlorotoluene and a 4-bromotoluenehigher potential-overcharge additive were respectively added in anamount of 1.0% by weight and 1.0% by weight and a lowerpotential-overcharge additive was not added.

COMPARATIVE EXAMPLE 17

A rechargeable lithium battery cell was fabricated by the same procedureas Example 11, except that a 4-fluorotoluene and a 4,4′-(hexafluoroisopropylidene)diphenol higher potential-overcharge additive wererespectively added in an amount of 1.0% by weight and 1.0% by weight andthe lower potential-overcharge additive was not added.

COMPARATIVE EXAMPLE 18

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 4-phenyl-1,3-dioxane and a4,4′-(hexafluoro isopropylidene)diphenol higher potential-overchargeadditive were respectively added in an amount of 1.0% by weight and 1.0%by weight and a lower potential-overcharge additive was not added.

COMPARATIVE EXAMPLE 19

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a bisphenol A diacetate and bisphenol Alower potential-overcharge additive were respectively added in an amountof 1.0% by weight and 1.0% by weight and a higher potential-overchargeadditive was not added.

COMPARATIVE EXAMPLE 20

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 1,1-diphenylethylene and bisphenol Alower potential-overcharge additive were respectively added in an amountof 1.0% by weight and 1.0% by weight and a higher potential-overchargeadditive was not added.

COMPARATIVE EXAMPLE 21

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 4-chlorotoluene higherpotential-overcharge additive was respectively added in an amount of 25%by weight and a lower potential-overcharge additive was not added.

COMPARATIVE EXAMPLE 22

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 1,1-diphenylethylene lowerpotential-overcharge additive was added in an amount of 25% by weightand a higher potential-overcharge additive was not added.

COMPARATIVE EXAMPLE 23

A rechargeable lithium battery cell was fabricated by the same procedureas in Example 11, except that a 4-chlorotoluene higherpotential-overcharge additive was added in an amount of 20% by weightand a 1,1-diphenylethylene lower potential-overcharge additive was addedin an amount of 5% by weight.

The cells fabricated by procedures according to comparative Examples 16to 23 (battery capacity 1 C=790 mAh) were constant current and constantvoltage charged at 158 mA and 4.2V, and then allowed to stand for 1 hourfollowed by discharging at 395 mA and 2.75V and being allowed to standfor 1 hour. These procedures were repeated 3 times, and the resultingcells were then charged at 395 mA and 4.2V for 3 hours.

Subsequently, each set of 20 cells fabricated by procedures according toExamples 16 to 23 were constant current and constant voltage overchargedat 1 (790 mAh) by 12V at room temperature (25° C.) for 2.5 hours. Thecells were monitored, and the results are shown in Table 7.

Further, these charged cells were allowed to stand at 85° C. for 4 days,and the discharge capacity was measured. The recovery rate was expressedby comparing the discharge capacity at 0.5 C after the cell was allowedto stand at the high temperature for 4 days as a percentage of thedischarge capacity at 0.5 C before the 4 days of high temperaturestorage.

The cycle life test was performed by constant current/constant voltagecharging at 1 C at 4V under a cut-off of 0.1 C, and discharging at 1 Cunder a cut-off voltage of 3.0V, for 300cycles. The capacity retentionwas expressed by comparing the discharge capacity after 300 cycles as apercentage of the discharge capacity at 1 cycle. The cycle-life test wascarried out under both a high temperature (45° C.) and at roomtemperature (25° C.).

In Table 7, the number before “L” means the number of test cells, andthe conditions for measuring the overcharge safety were identical tothose of the Table 2.

TABLE 7 Capacity First additive Second additive Capacity retention at(lower (higher retention room potential- potential- LSV Retention at 45°C. for temperature overcharge overcharge decomposition recovery 300cycles for 300 cycles additive) additive) start voltage (V) Overchargerate (%) (%) (%) Comparative — 4-chlorotoluene/ 4.73 12L0, 8L5 95 76 91Example 4-bromotoluene 16 Comparative — 4-fluorotoluene/ 4.72 15L0, 5L490 77 86 Example 4,4′- 17 (hexafluoro isopropylidene) diphenolComparative — 4-phenyl-1,3- 4.74 14L0, 6L4 85 78 85 Example dioxane/4,4-18 cyclohexylidene bisphenol Comparative bisphenol A 4.25 20L0 42 32 43Example diacetate/ 19 bisphenol A Comparative 1,1-diphenyl 4.22 20L0 4225 34 Example ethylene/ 20 bisphenol A Comparative — 4-chlorotoluene4.73 19L0, 1L4 53 43 49 Example 21 Comparative 1,1-diphenyl — 4.26 20L02 0 0 Example ethylene 22 Comparative 1,1-diphenyl 4-chlorotoluene 4.4720L0 15 0 0 Example ethylene 23

As shown in Table 7, cells according to Comparative Examples 17 and 18including only a higher potential-overcharge additive exhibiteddeteriorated overcharge characteristics, but cells according toComparative Examples 19 and 20 including only a lowerpotential-overcharge additive exhibited both deteriorated cycle-lifecharacteristics and deteriorated storing characteristics.

Further, cells according to Comparative Examples 21 to 23 including 25wt % of a higher potential-overcharge additive, a lowerpotential-overcharge additive, or a mixture thereof, deteriorated on thestorage characteristics and the cycle-life characteristics.

As described above, an electrolyte for a rechargeable lithium batteryaccording to the present invention including a higheroxidation-potential additive and a lower oxidation-potential additivecan provide a battery with improved safety during high temperaturestorage with good cycle-life characteristics.

1. An electrolyte for a rechargeable lithium battery, comprising: a first additive having an oxidation potential from 4.1 to 4.6V, wherein the first additive comprises a compound selected from the group consisting of Formula 1, Formula 2, and combinations thereof:

where R1 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; R4 is selected from the group consisting of: C₁-C₆ alkyl groups,

 where R6 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups, and —OR7 where R7 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; and R5 is selected from the group consisting of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups; a second additive having an oxidation potential from 4,4 to 5.OV; a non-aqueous organic solvent; and a lithium salt.
 2. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the first additive has an oxidation potential from 4.1 to 4.4V.
 3. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the first additive has an oxidation potential from 4.2 to 4.4V.
 4. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive has an oxidation potential from 4.6 to 5.0V.
 5. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive has an oxidation potential from 4.7 to 4.9V.
 6. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the oxidation potential of the first additive is from 0.1 to 0.7V less than the oxidation potential of the second additive.
 7. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the oxidation potential of the first additive is from 0.3 to 0.7V less than the oxidation potential of the second additive.
 8. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the electrolyte has an oxidation potential from 4.4 to 4.9V.
 9. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the electrolyte has an oxidation potential from 4.5 to 4.7V.
 10. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the first additive comprises a compound selected from the group consisting of Formulae 2 to 4, and combinations thereof:

where R5 is selected from the group consisting of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups; R1 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; R6 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; and R7 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups.
 11. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the first additive comprises a compound of Formula
 2. 12. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive comprises at least one compound selected from the group consisting of compounds represented by Formulae 5 to 9:

where X is selected from the group consisting of Cl, Br, and F; R9 is selected from the group consisting of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups; R2 is selected from the group consisting of C₁-C₆ alkyl groups, alkoxy groups, and alkylene groups; R3 is selected from the group consisting of C₁-C₆ alkyl groups C₂-C₆ alkylene groups; R10 is selected from the group consisting of C₁-C₆ alkyl groups, and A is selected from the group consisting of SO₂, CH₂, C═O, and CF₂.
 13. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive comprises at least one compound selected from the group consisting of compounds represented by Formulae 5, and 10 to 13:

where X is selected from the group consisting of Cl, Br, and F; and each of R9, R11, R12, and R13 is independently selected from the group consisting of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups.
 14. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive is represented by Formula 5:

selected from the group consisting of Cl, Br and F; and R9 is selected from the group of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups.
 15. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the combined amount of the first additive and the second additive is 0.01 to 20% by weight based on the total weight of the non-aqueous organic solvent and the lithium salt.
 16. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the combined amount of the first additive and the second additive is 0.1 to 10% by weight based on the total weight of the non-aqueous organic solvent and the lithium salt.
 17. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the combined amount of the first additive and the second additive is 5 to 10% by weight based on the total weight of the non-aqueous organic solvent and the lithium salt.
 18. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the second additive and the first additive are mixed at a weight ratio of 20: 0.1 to 3:2.
 19. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the electrolyte comprises 0.1 to 5% by weight of the first additive and 1 to 20% by weight of the second additive.
 20. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the non-aqueous organic solvent is selected from the group consisting of carbonates, esters, ethers, ketones and combinations thereof.
 21. The electrolyte for a rechargeable lithium battery according to claim 20, wherein the non-aqueous organic solvent is a carbonate selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate and combinations thereof.
 22. The electrolyte for a rechargeable lithium battery according to claim 20, wherein the non-aqueous organic solvent comprises a mixed solvent of cyclic carbonates and chain carbonates.
 23. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the non-aqueous organic solvent is a mixed solvent of a carbonate solvent and an aromatic hydrocarbon organic solvent.
 24. The electrolyte for a rechargeable lithium battery according to claim 23, wherein the aromatic hydrocarbon organic solvent is a compound represented by Formula 14:

where R14 is selected from the group consisting of halogens and C₁-C₁₀ alkyl groups and n is an integer from 0 to
 6. 25. The electrolyte for a rechargeable lithium battery according to claim 23, wherein the aromatic hydrocarbon organic solvent is selected from the group consisting of benzene, fluorobenzene, chlorobenzene, nitrobenzene, toluene, fluorotoluene, trifluorotoluene, xylene and mixtures thereof.
 26. The electrolyte for a rechargeable lithium battery according to claim 23, wherein the carbonate solvent and an aromatic hydrocarbon organic solvent are mixed in a volume ratio between 1:1 and 30:1.
 27. The electrolyte for a rechargeable lithium battery according to claim 1, wherein the lithium salt is selected from the group consisting of LiPF₆, LiBF₄, LiSbF₆, LiAsF₆, LiClO₄, CF₃SO₃Li, LiN(SO₂CF₃)₂, LiC₄F₉SO₃, LiAIO₄, LiAlOCI₄, LiN(SO₂C₂F₅)₂), LiN(C_(x)F_(2x+1)SO₂)(C_(y)F_(2y+1)SO₂) where x and y are natural numbers, LiCl, LiI and combinations thereof.
 28. An electrolyte for a rechargeable lithium battery, comprising: a first additive having an oxidation potential of 4.1 to 4.5V, wherein the first additive comprises a compound selected from the group consisting of Formula 1, Formula 2, and combinations thereof:

where R1 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; R4 is selected from the group consisting of: C₁-C₆ alkyl groups,

 where R6 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups, and —OR7 where R7 is selected from the group consisting of hydrogen, C₁-C₆ alkyl groups, and C₂-C₆ alkylene groups; and R5 is selected from the group consisting of C₁-C₆ alkyl groups and C₂-C₆ alkylene groups; a second additive having an oxidation potential from 4.4 to 5.0V; a non-aqueous organic solvent; and a lithium salt, wherein the electrolyte has an oxidation potential from 4.4 to 4.9V.
 29. The electrolyte for a rechargeable lithium battery according to claim 28, wherein the electrolyte has an oxidation potential from 4.5 to 4.7V. 